The present invention relates to a system for precisely relating a machine operation to the position of images on a moving web, and, particularly, to a system for precisely relating a cutoff operation to images on a moving web in a web-fed printing press system.
In a web-fed printing press, a web of material, typically paper, is fed from a storage mechanism, such as a reel stand, to one or more printing units which imprint the web with images (signatures). The imprinted web is then typically driven through respective processing units such as a dryer unit and/or coating equipment. The web is then fed to a cutting apparatus for separating the respective repeating signatures on the web. The cutting apparatus typically comprises a pair of cooperating cutting cylinders bearing one or more cutting blades. The cutting cylinders are rotated in synchronism with the printing units so that the blades intersect the moving web at predetermined points, e.g., between the repeating signatures (images). It is necessary that the cutting blade intersect the moving web on a repetitive basis in precise coordinated relationship with the repetition of the imprinted signatures on the web. However, various conditions of the printing system, such as, for example, web tension, splices and influence from folders, slitters, imprinters, gluers and other processing equipment cause the linear position of the web, and thus the signatures, to vary over time with respect to the cutting apparatus. Accordingly, it is necessary to periodically adjust the positional relationship of the web and cutting mechanism by advancing or retarding the linear position of the web with respect to the cutting apparatus.
Accordingly, an adjustment mechanism is typically provided to vary the linear position of the web relative to the cutting mechanism, i.e., the effective length of the web path from the printing unit to the cutting mechanism. For example, a compensation roller and a pair of cooperating idler rollers are often interposed in the web path upstream of the cutting mechanism. The relative position of the compensation roller with respect to the idler rollers is varied to change the effective length of the web path and thus advance or retard the relative position of the cutting mechanism to the repeating images on the web. A compensation motor is utilized to selectively adjust the position of the compensation roller.
In general, closed loop systems for controlling the adjustment (compensation) mechanism, and thus the linear position of the web image pattern relative to the cutting mechanism, are known. In such systems, an encoder is coupled to the cutting mechanism to provide respective pulses representative of the cutting mechanism operational cycle: a first pulse indicative of a nominal beginning (top dead center (TDC)) of each cutting cycle, and a second sequence of pulses indicative of incremental advances in the cutting cycle (e.g., 2500 incremental pulses per cutting cycle). The operator initializes the system by establishing a "window" of preset width corresponding to the portion of the cutting cycle during which the blade is intended to intersect the web, i.e., a window (capture range) of a length equal to a first predetermined number of incremental pulses, beginning a second predetermined number of incremental pulses after the top dead center pulse (nominal beginning of the cycle).
An optical scanner is disposed over the moving web between the compensation mechanism and the cutting mechanism, and projects a bar of light on the portion of the web instantaneously underlying the scanner. Images on the web reflect varying amounts of light in accordance with the density (darkness) of the image. The scanner receives the reflected light and generates an output signal indicative of the image density. The density signal is then compared to a reference signal representative of a predetermined threshold density. If a transition from low density (light) to high density (dark) of sufficient magnitude is detected (i.e., the predetermined threshold is crossed), within the predetermined capture range window, the transition point (the number of incremental pulses after top dead center at which the transition occurs) is compared to a count corresponding to the center of the window, and the compensation roller position advanced or retarded accordingly.
Such systems, however, are disadvantageous in that they require the operator to manually align the capture range window with a particular density transition to be monitored (cutmark). In addition, such systems are incapable of discriminating between the desired cutmark and other density transitions on the web which exceed the threshold value. Accordingly, system disruptions can cause conditions whereby the system erroneously locks on a density transition other than the intended cutmark. In such an event, or in the event that the cutmark is not detected within the capture range window, the operator is required to manually override the system (and position the compensation roller to realign the system with the intended cutmark). It is also necessary, in such systems, to maintain alignment between the scanner and the lateral position of the cutmark. Thus, such systems are particularly susceptible to loss of track due to lateral movement of the web, and, further, the position of the scanner must be manually changed in order to accommodate webs of differing widths.
Moreover, choice of a proper threshold level in such systems presents something of a dilemma. If the threshold is not set sufficiently high, the system tends to be susceptible to spurious triggering, and locking on density transitions other than the intended cutmark, and thus, erratic compensation or jitter. Conversely, if the density threshold is set too high, the images upon which the system is capable of operating becomes unduly limited. For example, a high density threshold tends to prevent the system from operating upon images that have not achieved full density. Further, in many instances the images on the web do not provide a density transition which is of sufficient magnitude, sufficiently isolated from other transitions, sufficiently large, and sufficiently linear in disposition to operate as a cutmark. In such cases, the printing of an extraneous cutmark, separate and apart from the image, is required. The extraneous cutmark is typically disposed in the lateral margin of the web, or between successive images. In either case, the use of the extraneous cutmark requires a surrounding clear space on the web and tends to increase wastage.